Cdim7 chord: diminished seventh
Cdim7 is the diminished seventh chord on C: C, Eb, Gb and Bbb (1–b3–b5–bb7). Cdim7 stacks the root C, the minor third Eb, the diminished fifth Gb and the diminished seventh Bbb. The diminished seventh stacks three minor thirds (1–b3–b5–bb7), forming a symmetrical and highly tense chord.
Notes in Cdim7: C – Eb – Gb – BbbFormula: 1 – b3 – b5 – bb7In the formula, the numbers count steps of the major scale starting from the note that names the chord; "b" lowers a half step, "#" raises one — and a slash names the bass note.
It can stand in for a dominant with the root omitted.
The same chord on all four instruments
Where to use it
Between two neighbouring chords it acts as a half-step stair: in C → C#° → DmDmDmDm it is the little step that pushes the music forward. In practice it is a passing chord — you barely notice it, but you feel the motion.
Part of the Sevenths family.